The present invention relates to inorganic composite materials of the class generally known as fiber-reinforced ceramic matrix composite materials. The invention includes silicon nitride fibers for such composites which are provided with protective coatings, and improved ceramic composites incorporating such protective coatings.
Fiber-reinforced ceramic matrix composites comprising glass-ceramic matrices are well known. U.S. Pat. No. 4,615,987 discloses silicon carbide fiber reinforced glass-ceramic composites wherein the matrix consists of an alkaline earth aluminosilicate glass-ceramic composition. Similar silicon-carbide-reinforced composites wherein the matrix consists of a barium-modified magnesium aluminosilicate glass-ceramic are reported in U.S. Pat. No. 4,589,900, while U.S. Pat. No. 4,755,489 discloses SiC-reinforced glass-ceramics wherein the glass-ceramic matrix contains excess Al.sub.2 O.sub.3 and consists predominantly of anorthite in combination with mullite or alumina.
Prospective uses for fiber-reinforced ceramic matrix composites such as described in these and other prior patents and literature include use as a structural element in high temperature environments such as heat engines. Thus the materials to be employed must not only exhibit good strength and toughness at ambient temperatures, but must also retain those desirable physical properties at the elevated temperatures encountered in the operating environment. Temperatures in the range of 700.degree.-1000.degree. C. and highly oxidizing conditions (due to the high-temperature activity of oxygen) are considered representative of such operating conditions.
Certain applications for ceramic matrix composites are anticipated that will require low dielectric constant and loss factor in addition to the high strength and toughness that these materials have traditionally offered. For these applications, lossy carbon or carbide materials cannot be employed, and thus alternative reinforcing materials must be found. One approach, disclosed in U.S. Pat. No. 4,711,860, involves the strengthening of cordierite glass-ceramics with silicon nitride (Si.sub.3 N.sub.4) whiskers, i.e., very short (&lt;500 .mu.m) length fibrous silicon nitride. However the high-temperature (1200.degree. C.) strengths achieved averaged below 50,000 psi.
An alternative candidate reinforcing material for low loss composites would be silicon nitride fiber. Fibers and/or chopped fibers of silicon nitride with lengths in the millimeter to meter range or longer are available. Unfortunately, silicon nitride reinforcing fibers have thus far provided neither strong composites nor low loss composites of minimum acceptable strength in oxide matrix systems.
The substitution of silicon nitride fibers for silicon carbide fibers in a glass-ceramic matrix material typically provides a composite of unacceptably low strength. We attribute this result to fiber embrittlement and/or strong interfacial bonding caused by interactions occurring between the oxide matrix and the nitride fibers at the high temperatures needed for full consolidation of these composites. Similarly, high dielectric losses have been seen in some nitride-reinforced systems where binder burnout processing was insufficient to completely remove carbonaceous binders from the composite preforms.
It is known to provide coatings on fiber reinforcement to be incorporated in composite materials in order to modify the behavior of the materials or the fibers therein. For example, U.S. Pat. No. 4,642,271 suggests boron nitride coatings for silicon carbide and other fiber reinforcement for ceramic matrix materials such as SiO.sub.2, ZrO.sub.2, mullite, and cordierite. In SiO.sub.2 systems, high-temperature strength and toughness were improved utilizing SiC reinforcing fibers coated with BN, although this effect was not observed for all fibers in all matrix systems.
Other coating systems and coating/matrix combinations are also known. U.S. Pat. No. 4,376,804, for example, describes carbon fibers coated with a metal oxide film intended to improve fiber adhesion and wetting by a molten metallic matrix material. U.S. Pat. No. 4,397,901 describes a composite wherein a woven or non-woven fiber substrate of carbon fibers is provided with a multi-layer carbon/silicon/silicon carbide coating to provide a composite article resistant to corrosive conditions. U.S. Pat. No. 4,405,685 describes a similar coating system for carbon fibers wherein an inner coating of carbon and a selected metal carbide with an outer coating of the metal carbide are used. This coating system is intended to provide enhanced fiber protection for fibers to be embedded in ceramic or, particularly, in metal matrix materials.
U.S. Pat. No. 4,481,257 discloses silicon carbide monofilaments coated with boron or boron carbide and exhibiting improved strength and bonding when used with metal or epoxy matrix materials. U.S. Pat. No. 4,485,179 describes the use, in a ceramic matrix composite comprising silicon carbide fibers, of an agent added to the matrix to reduce interaction with the silicon carbide fibers. Tantalum or niobium compounds are useful for this purpose.
While the foregoing patents and literature indicate a general interest in the development of coatings for fibers to be employed for the reinforcement of composite glass, metal and ceramic materials, no coating system has yet been developed which effectively protects and preserves the theoretical strengthening capacity of silicon nitride fibers in oxide ceramic matrix materials. Nor has any procedure for making high-strength nitride-fiber-reinforced ceramic composites been developed wherein the potential low dielectric loss characteristics of nitride-oxide materials are effectively preserved.
It is therefore a principal object of the present invention to provide a fiber-reinforced ceramic matrix composite comprising silicon nitride reinforcing fibers which exhibits improved strength and resistance to embrittlement under adverse high temperature conditions.
It is a further object of the invention to provide a method for making silicon nitride-reinforced glass-ceramic matrix composites which provides products of improved strength and reduced dielectric loss.
It is a further object of the invention to provide novel and improved protective coating systems for silicon nitride fibers utilized for ceramic matrix reinforcement, particularly glass-ceramic matrix reinforcement, and coated fibers incorporating such coatings.
Other objects and advantages of the invention will become apparent from the following description thereof.